There exists an increasing, need to generate very large quantities of oxygen through the cryogenic separation of air. For example in some gasification projects upwards of between about 10,000 and about 15,000 metric tons per day of oxygen are required. Typically, as plant production size increases the associated distillation column diameter is also increased to be able to distill a larger mass flow rate of air. In this regard, typically distillation diameter increases in proportion to the square root of plant capacity. However, there are practical limitations on column diameter given the fact that distillation columns are typically fabricated off site and shipped to their destination.
When column diameters are in a range of between about 6.0 and 6.5 meters, shipping limitations arise. The consequence of this is that the oxygen production capability of a single cryogenic air separation plant that is greater than about 5,000 metric tons per day becomes very impractical. Due to this sizing constraint, parallel air separation plants are fabricated. However, the construction of additional columns for such air separation plants carries with it a considerable expense.
More specifically, large quantities of oxygen are produced within cryogenic air separation plants that employ double column arrangements of a higher pressure column and a lower pressure column. In such a plant, the air is compressed, purified and cooled to a temperature suitable for its distillation. The air is then introduced into the higher pressure column. Within the higher pressure column, the introduction of the air produces an ascending vapor phase that becomes evermore rich in nitrogen and a descending liquid phase that becomes evermore rich in oxygen. At the top of the high pressure column, a nitrogen-rich vapor column overhead is produced that is condensed to initiate the formation of the descending liquid phase. Additionally, a stream of the condensate is used to reflux the lower pressure column and initiate a descending liquid phase within such column.
Within the higher pressure column, a kettle liquid or a crude-liquid oxygen is produced that is introduced into the lower pressure column for further refinement. This produces an oxygen-rich column bottoms from which a stream may be taken as an oxygen product. The higher and lower pressure columns may be thermally linked by a condenser-reboiler that can be located at or near the base of the lower pressure column to condense the nitrogen-rich vapor overhead of the higher pressure column against vaporizing the oxygen-rich liquid.
In the double column arrangement, above the point at which the crude-liquid oxygen or kettle liquid is introduced, a limitation or bottleneck is produced in which for a given column size, any increase in mass flow rate of the air feed to the plant will cause the column to flood. Thus, for maximum column diameter of between about 6 and about 6.5 meters, the production of an oxygen product is limited to about 5,000 metric tons per day.
In the prior art, there have been integrations involving two separate cryogenic air separation plants with the object of increasing the production of a product produced by the cryogenic air separation plants. For example, in U.S. Pat. No. 6,666,048 an integration is shown in which a single column nitrogen generation plant is integrated with a double column oxygen producing plant by introducing a waste stream into the incoming air stream. In the single column nitrogen generator, a stream of the column bottoms that is rich in oxygen is introduced into a heat exchanger that is used to condense reflux for such column. The resulting vaporized stream produces the waste stream. However, while this may increase the flow of air into the double column, the resulting plant is not debottlenecked because the same limitation with respect to the flow above the point of introduction of the kettle liquid still exists. Consequently, the degree of increase in the oxygen production that can be obtained from such integration is very limited.
As will be discussed, the present invention provides an integration of two cryogenic air separation plants in which oxygen production can be increased to a larger extent than is possible in the prior art and also in a manner that allows energy savings to be realized.